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Design >> Analog Design >> LDO AC small signal analysis
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Message started by pommie on Oct 10^th, 2007, 11:50pm

Title: LDO AC small signal analysis
Post by pommie on Oct 10^th, 2007, 11:50pm

Hello All,

I would like to be able to use the available small signal AC analysis to evaluate any given OTS(Off the shelf) LDO for its major datasheet electrical characteristics such as ;

1. Output noise- VONOISE- nV/Hz^2
2. PSRR
3. Gain Band Width of the error amplifier
4. open Loop Gain

Would any of you experts out there be aware of such a once stop test method?I am kicking the tires here with a discrete LDO design based on a LinTech LT1490 opamp.This is to familiarise myself with the process of AC loop response analysis and is purely educational. I am injecting the AC signal in to the inverting pin of the Opamp instead of the traditional injection point on the Non-inverting pin through the AC coupling cap which is AC short,DC open. And I am looking at the VDIV as the one Femto Henry INductor is DC short,AC open and that ways,my belief is that I still have the AC path for the feedback loop and I can affect the loop compensation by varying capacitor C5. But I notice that my Opamp is not biased properly any more. I have the schematics attached herewith. If you notice anything abnormal please let me know. It is needless to say this is my first attempt at SPICE modeling and any help in any form would be greatly appreciated!.

Thanks again,
AnalogNoob

Title: Re: LDO AC small signal analysis
Post by MonteCarlo on Oct 13^th, 2007, 3:55am

Is the pnp emitter connected to anything? Even if it is, I suspect the pnp transistor model is misbehaving as there is nothing else which could generate -41V on the output. Try a different pnp, pmos or voltage controlled current source.

BTW, your 1 femto Henry inductor is not an AC open as you stated. Nor is your 1 femto Farad capacitor an AC short/

Your overall goal of one test method for all 4 parameters is ambitious. Post your results if you make it work as I'd love to know how it's done.

-Monte